Hard carbon, method for producing the same, secondary battery containing the same, and electric device

Abstract

The application provides a hard carbon, a preparation method of the hard carbon, a secondary battery containing the hard carbon, and an electric device. In a nitrogen adsorption isotherm measured at a temperature of 77K, a total amount of nitrogen adsorption when a relative pressure P / P0 of nitrogen is between 10 ‑8 and 0.035 is V1 cm 3 (STP) / g, a total amount of nitrogen adsorption when the relative pressure P / P0 of nitrogen is between 0.035 and 1 is V2 cm 3 (STP) / g, and the hard carbon satisfies V2 / V1≤0.20 and 20≤V1≤150, wherein P represents a real pressure of nitrogen, and P0 represents a saturated vapor pressure of nitrogen at a temperature of 77K. The application can improve the capacity and the first coulomb efficiency of the hard carbon simultaneously.

Description

Technical Field

[0001] The present application belongs to the field of battery technology, and specifically relates to hard carbon and a preparation method thereof, a secondary battery containing the same, and an electrical device. Background Art

[0002] In recent years, secondary batteries have been widely used in energy storage systems such as hydropower, thermal, wind, and solar power stations, as well as in power tools, electric bicycles, electric motorcycles, electric vehicles, military equipment, aerospace, and other fields. With the application and promotion of secondary batteries, their energy density, service life, and rate performance have received increasing attention. Graphite is the most commonly used negative electrode active material for secondary batteries, but its theoretical gram capacity is only 372 mAh / g, which limits the potential for energy density improvement. Furthermore, the small interlayer spacing of graphite also limits rate performance improvements. As a new type of negative electrode active material, hard carbon has great potential for development because it can rapidly embed and extract active ions during the charge and discharge process of secondary batteries. However, hard carbon has low capacity and initial coulombic efficiency, which limits its potential to improve the energy density, service life, and rate performance of secondary batteries. Summary of the Invention

[0003] The purpose of this application is to provide a hard carbon and a preparation method thereof, a secondary battery containing the same, and an electrical device, aiming to simultaneously improve the capacity and first coulombic efficiency of the hard carbon.

[0004] The first aspect of the present application provides a hard carbon, wherein in a nitrogen adsorption isotherm measured at a temperature of 77K, the nitrogen relative pressure P / P0 is 10 -8 The total amount of nitrogen adsorption between 0.01% and 0.035 is V1cm 3 (STP) / g, the total amount of nitrogen adsorbed when the relative pressure of nitrogen P / P0 is between 0.035 and 1 is V2cm 3 (STP) / g, the hard carbon satisfies: V2 / V1≤0.20, and 20≤V1≤150, wherein P represents the actual pressure of nitrogen, and P0 represents the saturated vapor pressure of nitrogen at a temperature of 77K.

[0005] Compared to currently commercialized hard carbon, the hard carbon provided by this application achieves both higher capacity and first coulombic efficiency. Although the mechanism is unclear, the inventors speculate that one possible reason is that the hard carbon provided by this application has a moderate number of internal defects and a unique pore structure, resulting in a large number of active ion storage sites and high utilization rates. Therefore, the hard carbon structure of this application facilitates the embedding, storage, and extraction of active ions, thereby achieving both higher capacity and first coulombic efficiency.

[0006] In any embodiment of the present application, 50≤V1≤150. Alternatively, 70≤V1≤150. When V1 is within the suitable range, the hard carbon provided by the present application can have both higher capacity and higher first coulombic efficiency.

[0007] In any embodiment of the present application, 0.05≤V2 / V1≤0.20. Alternatively, 0.08≤V2 / V1≤0.20. When V2 / V1 is within the suitable range, the hard carbon can have both higher capacity and higher first coulombic efficiency.

[0008] In any embodiment of the present application, 4≤V2≤30. Alternatively, 6≤V2≤30. When V2 is within the suitable range, the hard carbon can have both higher capacity and higher first coulombic efficiency, and also have excellent rate performance.

[0009] In any embodiment of the present application, in the Raman spectrum of the hard carbon, I d / I g is 1.20-1.32, I d represents the intensity of d-peak with Raman shift in the range of 1350±50 cm -1 , and I g represents the intensity of g-peak with Raman shift in the range of 1580±50 cm -1 . In this case, the hard carbon has moderate degree of order, and thus has both higher capacity and higher first coulombic efficiency, and also has excellent rate performance.

[0010] In any embodiment of the present application, in the X-ray diffraction spectrum of the hard carbon, the 2θ value corresponding to the 002 peak is between 22° and 24°.

[0011] In any embodiment of the present application, the volume particle size Dv50 of the hard carbon is 2 μm-15 μm; alternatively, 4 μm-8 μm.

[0012] In any embodiment of the present application, the volume particle size Dv90 of the hard carbon is 5 μm-25 μm; alternatively, 8 μm-15 μm.

[0013] When the volume particle size Dv50 and / or Dv90 of the hard carbon is within the suitable range, the active ion and electron transport performance can be improved, and thus the rate performance of the secondary battery can be further improved.

[0014] In any embodiment of the present application, the specific surface area of the hard carbon is less than or equal to 5 m 2 / g; alternatively, 0.5 m 2 / g-5 m 2 / g. When the hard carbon's specific surface area is within an appropriate range, it can simultaneously achieve higher capacity and higher first coulombic efficiency, while also exhibiting better rate performance. Furthermore, when the hard carbon's specific surface area is within an appropriate range, it can also provide a strong bond between the hard carbon and the binder, thereby improving the cohesion and adhesion of the negative electrode sheet, reducing the negative electrode's volume expansion during cycling, and improving the secondary battery's cycle performance.

[0015] In any embodiment of the present application, the powder compaction density of the hard carbon under a force of 50,000 N is 0.96 g / cm 3 -1.05g / cm 3 When the compaction density of hard carbon powder is within an appropriate range, the compaction density of the negative electrode sheet can be increased, thereby increasing the energy density of the secondary battery.

[0016] In any embodiment of the present application, the tap density of the hard carbon is 0.80 g / cm 3 -0.95g / cm 3 When the tap density of hard carbon is within an appropriate range, the compaction density of the negative electrode sheet can be increased, thereby increasing the energy density of the secondary battery.

[0017] The second aspect of the present application provides a method for preparing hard carbon, comprising the following steps: S10, providing a carbon source; S20, heat-treating the carbon source in an inert atmosphere at a first temperature T1 for a time t1 to obtain a first intermediate product; S30, heat-treating the obtained first intermediate product in an air atmosphere at a second temperature T2 for a time t2 to obtain a second intermediate product; S40, carbonizing the obtained second intermediate product in an inert atmosphere at a third temperature T3 for a time t3 to obtain hard carbon, wherein the hard carbon has a nitrogen relative pressure P / P0 of 10 in a nitrogen adsorption isotherm measured at a temperature of 77K. -8 The total amount of nitrogen adsorption between 0.01% and 0.035 is V1cm 3 (STP) / g, the total amount of nitrogen adsorbed when the relative pressure of nitrogen P / P0 is between 0.035 and 1 is V2cm 3 (STP) / g, the hard carbon satisfies: V2 / V1≤0.20, and 20≤V1≤150, wherein P represents the actual pressure of nitrogen, and P0 represents the saturated vapor pressure of nitrogen at a temperature of 77K.

[0018] The inventors surprisingly discovered that by simultaneously subjecting the carbon source to a low-temperature heat treatment in an inert atmosphere, a secondary low-temperature heat treatment in air, and a high-temperature carbonization treatment in an inert atmosphere, the resulting hard carbon exhibits both high capacity and high initial coulombic efficiency. The hard carbon preparation method provided herein is simple and suitable for commercial production.

[0019] In any embodiment of the present application, T1<T3, and T2<T3.

[0020] In any implementation manner of the present application, T1≤T2; optionally, T1<T2.

[0021] In any embodiment of the present application, T1 is ≤ 300° C. Optionally, T1 is 180° C.-300° C. When T1 is within a suitable range, a relatively large number of skeleton structures suitable for introducing suitable pore structures and stable and not prone to collapse can be retained.

[0022] In any embodiment of the present application, T2 is less than 400°C; alternatively, T2 is between 270°C and 380°C. When T2 is within a suitable range, a suitable and abundant pore structure can be formed from the inside to the outside of the carbon skeleton material particles obtained in S20, thereby improving the active ion and electron transport properties, thereby increasing the capacity and first coulombic efficiency of the hard carbon.

[0023] In any embodiment of the present application, T3 is 1000° C.-1600° C.; alternatively, T3 is 1000° C.-1400° C. When T3 is within a suitable range, the order of the hard carbon can be better improved, so that the hard carbon has both high capacity and high first coulombic efficiency.

[0024] In any embodiment of the present application, t1 is 4h-60h.

[0025] In any embodiment of the present application, t2 is 1h-12h.

[0026] In any embodiment of the present application, t3 is 1h-12h.

[0027] In any embodiment of the present application, after S20 and before S30, the preparation method further includes: S21, crushing the first intermediate product obtained in S20 to ensure that the first intermediate product can fully contact with air during subsequent processing.

[0028] In any embodiment of the present application, after S30 and before S40, the preparation method further includes: S31, crushing the second intermediate product obtained in S30. Optionally, the volume particle size of the crushed second intermediate product satisfies: Dv50 of 2 μm to 15 μm, and / or Dv90 of 5 μm to 25 μm.

[0029] In any embodiment of the present application, the preparation method further includes: S50, crushing the hard carbon obtained in S40 to meet the required particle size, so as to facilitate the preparation of negative electrode slurry and negative electrode sheet.

[0030] In any embodiment of the present application, the inert atmosphere is selected from one or more of nitrogen atmosphere and argon atmosphere.

[0031] In any embodiment of the present application, the carbon source includes one or more of polymers, resins, and biomass materials.

[0032] In any embodiment of the present application, the polymer includes one or more of polyaniline and polypyrrole.

[0033] In any embodiment of the present application, the resin includes one or more of phenolic resin and epoxy resin.

[0034] In any embodiment of the present application, the biomass material includes one or more of starch, glucose, fructose, maltose, sucrose, cellulose, hemicellulose, and lignin. Optionally, the starch includes one or more of cereal starch, potato starch, and legume starch.

[0035] A third aspect of the present application provides a secondary battery comprising a negative electrode plate, wherein the negative electrode plate comprises the hard carbon of the first aspect of the present application, or the hard carbon prepared according to the method of the second aspect of the present application.

[0036] A fourth aspect of the present application provides an electrical device comprising the secondary battery of the third aspect of the present application.

[0037] The electric device of the present application includes the secondary battery provided by the present application, and thus has at least the same advantages as the secondary battery. BRIEF DESCRIPTION OF THE DRAWINGS

[0038] To more clearly illustrate the technical solutions of the embodiments of the present application, the following briefly introduces the drawings required for use in the embodiments of the present application. Obviously, the drawings described below are only some embodiments of the present application. For those skilled in the art, other drawings can be obtained based on the drawings without inventive effort.

[0039] Figure 1 Six common adsorption isotherm types for solid materials are shown.

[0040] Figure 2 This is a schematic diagram of one embodiment of the secondary battery of the present application.

[0041] Figure 3 yes Figure 2 An exploded schematic diagram of an embodiment of a secondary battery.

[0042] Figure 4 It is a schematic diagram of an embodiment of a battery module of the present application.

[0043] Figure 5 It is a schematic diagram of an embodiment of the battery pack of the present application.

[0044] Figure 6 yes Figure 5 An exploded schematic diagram of an embodiment of a battery pack is shown.

[0045] Figure 7 This is a schematic diagram of an embodiment of an electric device including the secondary battery of the present application as a power source.

[0046] Figure 8 is a scanning electron microscope (SEM) image of the hard carbon prepared in Example 2.

[0047] Figure 9 This is the nitrogen adsorption isotherm of the hard carbon prepared in Example 2 measured at a temperature of 77K.

[0048] Figure 10 is the nitrogen adsorption isotherm of the hard carbon prepared in Comparative Example 1 measured at a temperature of 77K.

[0049] In the accompanying drawings, the drawings are not necessarily drawn to scale. DETAILED DESCRIPTION

[0050] Hereinafter, the embodiments of the hard carbon of the present application, its preparation method, secondary battery containing the same, and electrical device will be described in detail with appropriate reference to the accompanying drawings. However, there may be cases where unnecessary detailed descriptions are omitted. For example, there may be cases where detailed descriptions of well-known matters and repeated descriptions of actually the same structures are omitted. This is to avoid the following description from becoming unnecessarily lengthy and to facilitate understanding by those skilled in the art. In addition, the drawings and the following description are provided for those skilled in the art to fully understand the present application and are not intended to limit the subject matter described in the claims.

[0051] The ranges disclosed herein are intended to be "open" ranges, i.e., the upper and lower limits of the range are not included. The ranges are also intended to include any and all sub-ranges of the range, i.e., all combinations of any two of the range limits, unless otherwise indicated. For example, a range of "1 to 10" is intended to include any number from 1 to 10, including the end values in the range, e.g., 1 and 10, and any and all intermediate values, e.g., 3.14, 4.56 etc. In addition, the phrase "a range of "a to b" is intended to include the end values in the range, i.e., "a" and "b", and to exclude any and all values outside of the range, unless otherwise indicated. For example, the phrase "a range of 1 to 10" is intended to exclude any value of 11 or greater, as well as 0 or less.

[0052] All embodiments and optional embodiments of the present application can be combined with each other to form new technical solutions, and such technical solutions should be considered to be included in the disclosure of the present application, unless otherwise specified.

[0053] All technical features and optional technical features of the present application can be combined with each other to form new technical solutions, and such technical solutions should be considered to be included in the disclosure of the present application, unless otherwise specified.

[0054] Unless otherwise specified, all steps of the present application can be performed in sequence or randomly, and preferably in sequence. For example, the method comprises steps (a) and (b), which means that the method can comprise steps (a) and (b) performed in sequence, or steps (b) and (a) performed in sequence. For example, the method also comprises step (c), which means that step (c) can be added to the method in any order, for example, the method can comprise steps (a), (b) and (c), or steps (a), (c) and (b), or steps (c), (a) and (b), etc.

[0055] Unless otherwise specified, the "includes" and "contains" mentioned in the present application are open-ended, and can also be closed. For example, the "includes" and "contains" can mean that other components not listed can also be included or contained, or only the listed components can be included or contained.

[0056] Unless otherwise specified, the term "or" is used in this application to be inclusive. For example, the phrase "A or B" means "A, B, or both A and B." More specifically, the condition "A or B" is satisfied if any of the following conditions are met: A is true (or exists) and B is false (or does not exist); A is false (or does not exist) and B is true (or exists); or both A and B are true (or exist).

[0057] Unless otherwise specified, in this application, the term "active ions" refers to ions that can be intercalated and extracted between the positive and negative electrodes of a secondary battery, including but not limited to lithium ions, sodium ions, etc.

[0058] According to the definition of the International Union of Pure and Applied Chemistry (IUPAC), micropores refer to pores with a pore diameter of less than 2 nm, mesopores refer to pores with a pore diameter of 2 nm to 50 nm, and macropores refer to pores with a pore diameter of >50 nm. In the context of this application, the term "micropore" refers to pores with a pore diameter of <2 nm; the term "mesopore" refers to pores with a pore diameter of 2 nm to 50 nm; and the term "macropore" refers to pores with a pore diameter of >50 nm. In the context of this application, the terms "larger pores" and "smaller pores" are relative concepts.

[0059] Secondary batteries, also known as rechargeable batteries or storage batteries, are batteries that can be recharged to activate the active materials and continue to be used after the battery is discharged. Generally, secondary batteries include a positive electrode plate, a negative electrode plate, a separator, and an electrolyte. During the charge and discharge process of the battery, active ions are embedded in and released back and forth between the positive electrode plate and the negative electrode plate. The separator is set between the positive electrode plate and the negative electrode plate, mainly to prevent the positive and negative electrodes from short-circuiting, while allowing active ions to pass through. The electrolyte plays the role of conducting active ions between the positive electrode plate and the negative electrode plate. Currently, secondary batteries are widely used in energy storage power systems such as hydropower, thermal power, wind power, and solar power stations, as well as in power tools, electric bicycles, electric motorcycles, electric vehicles, military equipment, aerospace, and other fields.

[0060] With the application and promotion of secondary batteries, their energy density, service life, and rate performance are receiving increasing attention. The performance of the negative electrode active material determines, to a certain extent, the energy density, service life, and safety of secondary batteries. Graphite (including natural graphite and artificial graphite) is the most commonly used negative electrode active material for secondary batteries, but its theoretical gram capacity is only 372mAh / g, which means that there is very limited room for energy density improvement. Furthermore, the small interlayer spacing of graphite limits the improvement of rate performance, and it can no longer meet the actual demand for high-rate performance secondary batteries.

[0061] Hard carbon refers to carbon that is difficult to graphitize, even at temperatures above 2500°C. Hard carbon has a complex structure and is a type of carbon material that exhibits a graphite microcrystalline structure, in which the graphite microcrystals are irregularly arranged, the number of graphite microcrystalline layers is small, and the edges of the layers may be cross-linked. The hard carbon structure also includes amorphous regions, where the amorphous regions mainly include micropores, defects, sp3 hybridized carbon atoms, carbon chains, and some functional groups. Therefore, there may be multiple active ion storage sites in the hard carbon structure, such as the surface of graphite microcrystals, between graphite microcrystal layers, at the edges of graphite microcrystal layers, and in micropores.

[0062] Compared to graphite, hard carbon has a larger interlayer spacing and a richer microporous structure, which facilitates the storage, rapid embedding, and extraction of active ions. This enables secondary batteries to possess excellent low-temperature performance, power performance, and safety. Hard carbon offers unique advantages, particularly in the field of power batteries. However, most commercialized hard carbons are low-capacity, standard hard carbons with low capacity and initial coulombic efficiency (e.g., capacity typically ranges from 200 mAh / g to 280 mAh / g, and initial coulombic efficiency is typically below 80%), severely limiting their practical applications.

[0063] Therefore, how to simultaneously improve the capacity and first coulombic efficiency of hard carbon remains a technical challenge that needs to be solved urgently.

[0064] In view of this, a first aspect of the embodiments of the present application provides a hard carbon that takes into account both high capacity and first coulombic efficiency, and enables a secondary battery to have high energy density, long service life and excellent rate performance.

[0065] In the nitrogen adsorption isotherm of the hard carbon of the first embodiment of the present application measured at a temperature of 77K, the nitrogen relative pressure P / P0 is 10 -8 The total amount of nitrogen adsorption between 0.01% and 0.035 is V1cm 3 (STP) / g, the total amount of nitrogen adsorbed when the relative pressure of nitrogen P / P0 is between 0.035 and 1 is V2cm 3 (STP) / g, the hard carbon satisfies: V2 / V1≤0.20, and 20≤V1≤150, wherein P represents the actual pressure of nitrogen, and P0 represents the saturated vapor pressure of nitrogen at a temperature of 77K.

[0066] The K mentioned above is the standard Kelvin temperature unit, and 77K represents the temperature of liquid nitrogen.

[0067] When gas molecules move to the surface of a solid material, due to the interaction between the gas molecules and the molecules on the surface of the solid material (for example, van der Waals forces or chemical bonds, etc.), the gas molecules will temporarily stay on the solid surface, increasing the concentration of gas molecules on the surface of the solid material. This phenomenon is called adsorption of gas molecules on the surface of the solid material. The gas can be called the adsorbate, and the solid material can be called the adsorbent. The adsorption isotherm refers to the relationship curve between the concentrations of the adsorbate (for example, gas) in the two phases (for example, the interface between gas and solid material) when the adsorption process at a certain temperature reaches equilibrium at the interface between the two phases (for example, the interface between gas and solid material). Different pore structures of solid materials result in different types of adsorption isotherms. Figure 1 Six common adsorption isotherm types for solid materials are shown.

[0068] In the nitrogen adsorption isotherm of the hard carbon of the present application measured at a temperature of 77K, the nitrogen relative pressure P / P0 is 10 -8 Total nitrogen adsorption V1cm between 0.035 and 0.035 3 (STP) / g and the total amount of nitrogen adsorption V2cm between the relative pressure P / P0 of nitrogen 0.035 and 1 3 (STP) / g satisfies V2 / V1≤0.20. Therefore, the adsorption process of the hard carbon of the present application mainly occurs in the stage of relatively low pressure, that is, P / P0 is 10 -8 At the same time, the hard carbon of the present application is in a nitrogen relative pressure P / P0 of 10 -8 The total amount of nitrogen adsorption V1 at 20cm 3 (STP) / g to 150cm 3 (STP) / g.

[0069] Compared to currently commercialized hard carbon, the hard carbon provided by this application achieves both higher capacity and first coulombic efficiency. Although the mechanism is unclear, the inventors speculate that one possible reason is that the hard carbon provided by this application has a moderate number of internal defects and a unique pore structure, resulting in a large number of active ion storage sites and high utilization rates. Therefore, the hard carbon structure of this application facilitates the embedding, storage, and extraction of active ions, thereby achieving both higher capacity and first coulombic efficiency.

[0070] At present, in the nitrogen adsorption isotherm of commercial hard carbon measured at 77K, the nitrogen relative pressure P / P0 is between 10 -8 Total nitrogen adsorption V1cm between 0.035 and 0.035 3 (STP) / g and the total amount of nitrogen adsorption V2cm between the relative pressure P / P0 of nitrogen 0.035 and 1 3V2 / V1≤0.20. Possible reasons are that the commercial hard carbon has more internal defects, unreasonable pore structure or less microporous structure, which cannot provide more active ion storage sites, and the utilization rate of the only active ion storage sites is also low; at the same time, the commercial hard carbon may contain a high content of mesoporous or macroporous structure, thereby causing a high proportion of electrolyte infiltration area in the hard carbon. The inventors further found that the higher the ratio of V2 to V1 in the adsorption isotherm, the higher the proportion of electrolyte infiltration area in the hard carbon, the higher the first irreversible capacity loss of the hard carbon, and the lower the first coulombic efficiency of the hard carbon.

[0071] In addition, when the total nitrogen adsorption amount of the hard carbon at a nitrogen relative pressure P / P0 of 10 -8 to 0.035 is <20 cm 3 (STP) / g, the pore structure of the hard carbon is not developed and the specific surface area is too low, the hard carbon cannot provide more active ion storage sites, so that the gram capacity of the hard carbon is low; at the same time, due to the undeveloped pore structure of the hard carbon, the active ions stored in the storage sites are not easy to be removed, so that the first coulombic efficiency of the hard carbon is also low.

[0072] When the total nitrogen adsorption amount of the hard carbon at a nitrogen relative pressure P / P0 of 10 -8 to 0.035 is >150 cm 3 (STP) / g, the structure of the hard carbon becomes brittle, so that the internal pore structure is easily collapsed to cause pore merging, resulting in a high proportion of mesoporous or macroporous structure in the hard carbon, and thus causing a large amount of electrolyte to infiltrate. Therefore, the hard carbon can provide more active ion storage sites and provide a high gram capacity, but due to the too complex structure of the hard carbon, part of the active ions stored in the active ion storage sites cannot be removed, so that the first coulombic efficiency of the hard carbon is low.

[0073] The total nitrogen adsorption amount V1 of the hard carbon provided in the present application at a nitrogen relative pressure P / P0 of 10 -8 to 0.035 is >150 cm 3(STP) / g satisfies 20≤V1≤150. The higher V1 is, the more active ion storage sites the hard carbon provides, and the higher the capacity of the hard carbon is. In some embodiments, optionally, 30≤V1≤150, 40≤V1≤150, 50≤V1≤150, 60≤V1≤150, 70≤V1≤150, 80≤V1≤150, 90≤V1≤150, 100≤V1≤150, 110≤V1≤150, 120≤V1≤150,30≤V1≤140,40≤V1≤140,50≤V1≤140,60≤V1≤140,70≤V1≤140,80≤V1≤140,90≤V1≤140,100≤V1≤140,110≤V1≤140,120≤V1≤140,30≤V1≤130,40≤V1≤130,50≤V1≤130,60≤V1 ≤130, 70≤V1≤130, 80≤V1≤130, 90≤V1≤130, 100≤V1≤130, 110≤V1≤130, 30≤V1≤120, 40≤V1≤120, 50≤V1≤120, 60≤V1≤120, 70≤V1≤120, 80≤V1≤120, 90≤V1≤120, 100≤V1≤120, 110≤V1≤120. When V1 is within a suitable range, the hard carbon provided by the present application can simultaneously have higher capacity and first coulombic efficiency.

[0074] The hard carbon provided by the present application is in a nitrogen relative pressure P / P0 of 10 -8 Total nitrogen adsorption V1cm between 0.035 and 0.035 3 (STP) / g and the total amount of nitrogen adsorption V2cm between the relative pressure P / P0 of nitrogen 0.035 and 1 3(STP) / g satisfies V2 / V1≤0.20. Within this range, the hard carbon may have a suitable pore structure, can provide more active ion storage sites, and the utilization rate of the active ion storage sites is also high; at the same time, the pore structure inside the hard carbon is mainly microporous structure, and the content of mesoporous or macroporous structure is moderate, thereby reducing the electrolyte infiltration area inside the hard carbon. In some embodiments, V2 / V1 may be ≤0.19, ≤0.18, ≤0.17, ≤0.16, ≤0.15, ≤0.14, ≤0.13, ≤0.12, ≤0.11, or ≤0.10. At the same time, V2 / V1 should not be too low, at which point the active ions stored in the active ion storage sites will have a high difficulty in escaping. In some embodiments, optionally, 0.05≤V2 / V1≤0.20, 0.06≤V2 / V1≤0.20, 0.08≤V2 / V1≤0.20, 0.10≤V2 / V1≤0.20, 0.12≤V2 / V1≤0.20, 0.14≤V2 / V1≤0.20, 0.16≤V2 / V1≤0.20, 0.05≤V2 / V1≤0.18, 0.06≤V2 / V1≤0.18, 0.08≤V2 / V1 When V2 / V1 is within a suitable range, hard carbon can simultaneously have higher capacity and higher first coulombic efficiency.

[0075] In some embodiments, 0<V2≤30. When V2 is higher, the electrolyte infiltration area inside the hard carbon accounts for a higher proportion, and the first irreversible capacity loss of the hard carbon increases. At the same time, V2 should not be too low. When V2 is lower, it is difficult for active ions to quickly escape and embed, and the rate performance of the hard carbon may deteriorate. Optionally, 1≤V2≤30, 2≤V2≤30, 3≤V2≤30, 4≤V2≤30, 5≤V2≤30, 6≤V2≤30, 7≤V2≤30, 8≤V2≤30, 9≤V2≤30, 10≤V2≤30, 1≤V2≤25, 2≤V2≤25, 3≤V2≤25, 4≤V2≤25, 5≤V2≤25 , 6≤V2≤25, 7≤V2≤25, 8≤V2≤25, 9≤V2≤25, 10≤V2≤25, 1≤V2≤20, 2≤V2≤20, 3≤V2≤20, 4≤V2≤20, 5≤V2≤20, 6≤V2≤20, 7≤V2≤20, 8≤V2≤20, 9≤V2≤20, or 10≤V2≤20. When V2 is within a suitable range, hard carbon can have higher capacity and higher first coulombic efficiency, and also has excellent rate performance.

[0076] In some embodiments, the hard carbon simultaneously satisfies the following conditions: 0.08≤V2 / V1≤0.20, 70≤V1≤150, and 6≤V2≤30. In this case, the hard carbon can provide more active ion storage sites, so that it has a higher capacity; at the same time, the hard carbon has a suitable pore structure inside, which can facilitate the embedding and storage of active ions without hindering the release of active ions, so that the utilization rate of the active ion storage sites is also high; in addition, the pore structure inside the hard carbon is mainly microporous structure, with a moderate content of mesoporous or macroporous structure, which can also reduce the electrolyte infiltration area inside the hard carbon. Therefore, the hard carbon can have a higher capacity and a higher first coulombic efficiency, while also having excellent rate performance. Further, the hard carbon simultaneously satisfies the following conditions: 0.08≤V2 / V1≤0.20, 85≤V1≤140, and 7≤V2≤25.

[0077] In some embodiments, in the Raman spectrum of the hard carbon, I d / I g 1.20-1.32, I d Indicates that the Raman shift is 1350±50cm - d peak intensity within 1 range, I g Indicates that the Raman shift is 1580±50cm -1 The g peak intensity in the range of λ is 532 nm, where the laser wavelength λ is 532 nm.

[0078] The d peak is generated by the carbon atom lattice defect, and the g peak is generated by the in-plane vibration of the sp2 carbon atom. In the hard carbon structure, the d peak intensity is related to the number of hard carbon structure defects, and the g peak intensity is related to the number of graphite crystals in the hard carbon structure. Therefore, I d / I g It can characterize the order of hard carbon structure. d / I g The smaller the size, the higher the order of the hard carbon structure, the higher the integrity of the carbon plane, and the higher the first coulombic efficiency of the hard carbon, but the capacity becomes lower and the rate performance becomes worse. The hard carbon of this application meets I d / I g It is 1.20-1.32. At this time, the order degree of the hard carbon structure is moderate, so that the hard carbon has a higher capacity and a higher first coulombic efficiency, as well as excellent rate performance.

[0079] In some embodiments, in the X-ray diffraction spectrum of the hard carbon, the 2θ value corresponding to the 002 peak is between 22° and 24°.

[0080] In some embodiments, the volume particle size Dv50 of the hard carbon is 2 μm-15 μm; optionally, 4 μm-8 μm.

[0081] In some embodiments, the volume particle size Dv90 of the hard carbon is 5 μm-25 μm; optionally, 8 μm-15 μm.

[0082] In some embodiments, the hard carbon simultaneously satisfies a volume particle size Dv50 of 4 μm to 8 μm and a volume particle size Dv90 of 8 μm to 15 μm.

[0083] When the volume particle size Dv50 and / or Dv90 of the hard carbon is within an appropriate range, it is beneficial to improve the active ion and electron transport performance, thereby further improving the rate performance of the secondary battery.

[0084] In some embodiments, the specific surface area of ​​the hard carbon is less than or equal to 5 m 2 / g. Optionally, the specific surface area of ​​the hard carbon can be 0.1m 2 / g-5m 2 / g,0.5m 2 / g-5m 2 / g,1m 2 / g-5m 2 / g,1.5m 2 / g-5m 2 / g,2m 2 / g-5m 2 / g,2.5m 2 / g-5m 2 / g,3m 2 / g-5m2 / g,3.5m 2 / g-5m 2 / g,4m 2 / g-5m 2 / g,0.5m 2 / g-4m 2 / g,1m 2 / g-4m 2 / g,1.5m 2 / g-4m 2 / g,2m 2 / g-4m 2 / g,2.5m 2 / g-4m 2 / g,3m 2 / g-4m 2 / g,0.5m 2 / g-3m 2 / g,1m 2 / g-3m 2 / g,1.5m 2 / g-3m 2 / g, or 2m 2 / g-3m 2 / g. A lower specific surface area helps to reduce the surface activity of hard carbon and reduce the formation of solid electrolyte interface film (SEI film), thereby improving the first coulombic efficiency of hard carbon and secondary batteries; a higher specific surface area is conducive to accelerating the transmission of active ions, thereby improving the rate performance of secondary batteries. When the specific surface area of ​​hard carbon is within an appropriate range, hard carbon can have higher capacity and first coulombic efficiency at the same time, and also have better rate performance. In addition, when the specific surface area of ​​hard carbon is within an appropriate range, there can be a strong bonding force between hard carbon and the binder, which can improve the cohesion and adhesion of the negative electrode sheet, reduce the volume expansion of the negative electrode sheet during the cycle, and make the secondary battery have better cycle performance.

[0085] In some embodiments, the hard carbon powder compaction density under a force of 50,000 N is 0.96 g / cm 3 -1.05g / cm 3 When the compaction density of hard carbon powder is within an appropriate range, the compaction density of the negative electrode sheet can be increased, thereby increasing the energy density of the secondary battery.

[0086] In some embodiments, the tap density of the hard carbon is 0.80 g / cm 3 -0.95g / cm 3 When the tap density of hard carbon is within an appropriate range, the compaction density of the negative electrode sheet can be increased, thereby increasing the energy density of the secondary battery.

[0087] In this application, nitrogen adsorption testing of hard carbon at 77 K can be conducted in accordance with GB / T 21650.2-2008, "Determination of Pore Size Distribution and Porosity of Solid Materials by Mercury Intrusion and Gas Adsorption - Part 2: Analysis of Mesopores and Macropores by Gas Adsorption." For example, the measurement can be performed using a surface area and pore size analyzer, such as the ASAP2460 surface area and pore size analyzer from Micromeritics, Inc., USA.

[0088] In this application, the volume particle sizes Dv50 and Dv90 of hard carbon are generally known in the art and represent the particle sizes corresponding to the 50% and 90% cumulative volume distribution percentages, respectively. These can be measured using instruments and methods known in the art. For example, they can be conveniently measured using a laser particle size analyzer, such as the Mastersizer 2000E laser particle size analyzer from Malvern Instruments Ltd., UK, in accordance with the particle size distribution laser diffraction method specified in GB / T 19077-2016.

[0089] In this application, the specific surface area of ​​hard carbon is generally known in the art and can be measured using instruments and methods known in the art. For example, it can be measured using the nitrogen adsorption specific surface area analysis method according to GB / T 19587-2017 and calculated using the BET (Brunauer Emmett Teller) method. The nitrogen adsorption specific surface area analysis can be measured using a Micromeritics ASAP3020 surface area and pore size analyzer.

[0090] In this application, the compacted density of hard carbon powder is a well-known meaning in the art and can be measured using instruments and methods known in the art. For example, it can be measured using an electronic pressure testing machine (such as UTM7305) in accordance with the standard GB / T24533-2009. An exemplary test method is as follows: Weigh 1g of hard carbon powder and add a pressure gauge with a bottom area of ​​1.327cm 2 In the mold, the pressure is increased to 5000 kg (equivalent to 50000 N), the pressure is maintained for 30 seconds, and then the pressure is released and maintained for 10 seconds. Then the compaction density of the hard carbon powder under the force of 50000 N is recorded and calculated.

[0091] In this application, the tap density of hard carbon has a well-known meaning in the art and can be measured using instruments and methods known in the art, for example, referring to GB / T5162-2006 and using a powder tap density tester (e.g., Dandong Better BT-301).

[0092] Methods for preparing hard carbon

[0093] The second aspect of the embodiment of the present application provides a method for preparing hard carbon, comprising the following steps: S10, providing a carbon source; S20, heat-treating the carbon source in an inert atmosphere at a first temperature T1 for a time t1 to obtain a first intermediate product; S30, heat-treating the obtained first intermediate product in an air atmosphere at a second temperature T2 for a time t2 to obtain a second intermediate product; S40, carbonizing the obtained second intermediate product in an inert atmosphere at a third temperature T3 for a time t3 to obtain hard carbon. In the nitrogen adsorption isotherm measured at a temperature of 77K, the nitrogen relative pressure P / P0 of the hard carbon is 10 -8 The total amount of nitrogen adsorption between 0.01% and 0.035 is V1cm 3 (STP) / g, the total amount of nitrogen adsorbed when the relative pressure of nitrogen P / P0 is between 0.035 and 1 is V2cm 3 (STP) / g, the hard carbon satisfies: V2 / V1≤0.20, and 20≤V1≤150, wherein P represents the actual pressure of nitrogen, and P0 represents the saturated vapor pressure of nitrogen at a temperature of 77K.

[0094] The preparation method of the second aspect of the embodiment of the present application can prepare the hard carbon of any example of the first aspect of the embodiment of the present application.

[0095] In some embodiments, T1<T3, T2<T3.

[0096] In some embodiments, T1≤T2. Alternatively, T1<T2.

[0097] The inventors surprisingly discovered that after simultaneously subjecting the carbon source to a low-temperature heat treatment in an inert atmosphere, a secondary low-temperature heat treatment in air, and a high-temperature carbonization treatment in an inert atmosphere, the resulting hard carbon exhibits both high capacity and high first coulombic efficiency. Furthermore, compared to existing commercial hard carbons, the hard carbon prepared using the present method exhibits significantly improved capacity and first coulombic efficiency.

[0098] The preparation method of the hard carbon provided in this application is simple and suitable for commercial production.

[0099] The preparation method of hard carbon provided in the present application does not require the addition of additional conductive agents or other additives, so the hard carbon obtained by the preparation method provided in the present application has a lower heteroatom content, which can further reduce the irreversible consumption of active ions by heteroatoms.

[0100] In the preparation method of hard carbon provided in the present application, heat treating the carbon source in an inert atmosphere and a lower first temperature can better stabilize the framework structure of the carbon source and provide a stable carbon skeleton structure for the subsequent secondary low-temperature heat treatment and high-temperature carbonization treatment pore-forming process; the first intermediate product is subjected to a secondary low-temperature heat treatment in an air atmosphere and a lower second temperature, which can form a suitable pore structure in the carbon skeleton structure, facilitating the subsequent secondary pore-forming process of high-temperature carbonization treatment; the second intermediate product is subjected to a high-temperature carbonization treatment in an inert atmosphere and a higher third temperature, which can improve the order of the obtained hard carbon.

[0101] There is no particular limitation on the type of inert atmosphere. In some embodiments, the inert atmosphere is selected from one or more of nitrogen atmosphere and argon atmosphere.

[0102] In some embodiments, the first temperature T1 is ≤ 300°C. When the first temperature T1 is high, it is easy to cause a large amount of decomposition of the skeleton structure suitable for conversion into a suitable pore structure formed after the carbon source is heat treated, which leads to a lack of a suitable skeleton structure to form a pore structure during the subsequent heat treatment process. On the nitrogen adsorption isotherm of the hard carbon measured at a temperature of 77K, V1 and V2 are both small, and the capacity of the hard carbon is low. Furthermore, the first temperature T1 should not be too low. When the first temperature T1 is low, the degree of cross-linking after the carbon source is heat treated is low, and the skeleton structure formed is fragile and has low stability. When the pore structure is introduced by low-temperature heat treatment in an air atmosphere, the fragile skeleton structure is easy to collapse, which leads to a reduction in the pore structure of the hard carbon. On the nitrogen adsorption isotherm of the hard carbon measured at a temperature of 77K, V1 is small. At the same time, the collapse of the skeleton structure will cause pore merging, and the number and proportion of larger pores will increase, further reducing the capacity and first coulombic efficiency of the hard carbon.

[0103] In some embodiments, the first temperature T1 may be 180° C.-300° C., 200° C.-300° C., 220° C.-300° C., 240° C.-300° C., 260° C.-300° C., 280° C.-300° C., 180° C.-280° C., 200° C.-280° C., 220° C.-280° C., 240° C.-280° C., 260° C.-280° C., 180° C.-260° C., 200° C.-260° C., 220° C.-260° C., 240° C.-260° C., 180° C.-240° C., 200° C.-240° C., 220° C.-240° C., or 180° C.-220° C. When the first temperature T1 is within an appropriate range, a skeleton structure that is suitable for introducing a suitable pore structure and is stable and not easily collapsed can be retained.

[0104] In some embodiments, the heat treatment time t1 is 4 hours to 60 hours. Those skilled in the art can select an appropriate heat treatment time within the above range based on the first temperature used. For example, when the first temperature is high, the heat treatment time can be appropriately shortened. The heat treatment time also varies slightly depending on the type of carbon source, and those skilled in the art can adjust it according to actual conditions.

[0105] In some embodiments, the second temperature T2 is less than 400°C. When the second temperature T2 is too high, the carbon skeleton material obtained by S20 reacts with the air at a faster rate, and the air is insufficient to diffuse into the interior of the material particles, which causes the pore structure formed during the pore-forming process to tend to be concentrated in the surface area of ​​the particles; in addition, after the surface area of ​​the particles is excessively porous, the pore structure easily collapses, causing the pores to merge, resulting in a higher number and proportion of larger pores, which in turn leads to a decrease in the capacity and first coulombic efficiency of the hard carbon. Furthermore, the second temperature T2 should not be too low. When the second temperature T2 is low, the carbon skeleton material obtained by S20 reacts with the air at a lower rate, and the pore-forming effect is weak, resulting in a lower capacity of the hard carbon.

[0106] In some embodiments, the second temperature T2 may be 200°C-395°C, 220°C-395°C, 240°C-395°C, 260°C-395°C, 280°C-395°C, 300°C-395°C, 320°C-395°C, 340°C-395°C, 360°C-395°C, 200°C-380°C, 220°C-380°C, 24 0℃-380℃, 260℃-380℃, 270℃-350℃, 270℃-380℃, 280℃-380℃, 300℃-380℃, 320℃-380℃, 340℃-380℃, 360℃-380℃, 290℃-380℃, 290℃-350℃, 200℃-360℃, 220℃-360℃, 24 0℃-360℃, 260℃-360℃, 280℃-360℃, 300℃-360℃, 320℃-360℃, 200℃-340℃, 220℃-340℃, 240℃-340℃, 260℃-340℃, 280℃-340℃, 300℃-340℃, 200℃-320℃, 220℃-320℃, 2 40°C-320°C, 260°C-320°C, 280°C-320°C, 200°C-300°C, 220°C-300°C, 240°C-300°C, 260°C-300°C, 200°C-280°C, 220°C-280°C, 240°C-280°C, 200°C-260°C, 220°C-260°C, or 200°C-240°C. When the second temperature T2 is within a suitable range, a suitable and rich pore structure can be formed from the inside to the outside of the carbon skeleton material particles obtained in S20, thereby improving the active ion and electron transport properties, thereby further improving the capacity and first coulombic efficiency of the hard carbon.

[0107] In some embodiments, the heat treatment time t2 is 1 hour to 12 hours. Those skilled in the art can select an appropriate heat treatment time within the above range based on the second temperature used. For example, when the second temperature is higher, the heat treatment time can be appropriately shortened. Depending on the type of carbon source, the heat treatment time also varies slightly, and those skilled in the art can adjust it according to actual conditions.

[0108] In some embodiments, optionally, when T2≤270°C, t2≥10h; further, when 200°C≤T2≤270°C, 10h≤t2≤12h.

[0109] In some embodiments, optionally, when 270°C < T2 < 400°C, t2 ≤ 8h; further, when 270°C < T2 < 400°C, 1h ≤ t2 ≤ 8h.

[0110] In some embodiments, 270℃≤T2≤380℃, and 1h≤t2≤4h. When both conditions are met within the given ranges, the capacity and the first coulombic efficiency of the hard carbon can be further improved.

[0111] In some embodiments, the third temperature T3 is 1000℃-1600℃. When the third temperature T3 is lower, a large number of extremely small pore structures in the second intermediate product structure obtained in S30 are retained, but the strength of these extremely small pore structures is low, and they are easily destroyed in the later charging and discharging process, resulting in a decrease in the utilization rate of active ion storage sites, and thus a decrease in the capacity and the first coulombic efficiency of the hard carbon. Further, the third temperature T3 should not be too high. When the third temperature T3 is higher, the graphite crystallite structure formed in the carbonization process is prone to micro-motion, which forces the adjustment of the pore structure formed, resulting in a significant decrease in the pore structure and a decrease in the capacity of the hard carbon.

[0112] In some embodiments, the third temperature T3 can be 1100℃-1600℃, 1200℃-1600℃, 1300℃-1600℃, 1400℃-1600℃, 1500℃-1600℃, 1000℃-1500℃, 1100℃-1500℃, 1200℃-1500℃, 1300℃-1500℃, 1400℃-1500℃, 1000℃-1400℃, 1100℃-1400℃, 1200℃-1400℃, 1300℃-1400℃, 1000℃-1300℃, 1000℃-1350℃, 1100℃-1300℃, 1200℃-1300℃, 1000℃-1200℃, 1100℃-1200℃, or 1000℃-1100℃. When the third temperature T3 is within the appropriate range, the order of the hard carbon can be better improved, and the hard carbon can have both a high capacity and a high first coulombic efficiency.

[0113] In some embodiments, the heat treatment time t3 is 1h-12h. Those skilled in the art can select a suitable heat treatment time within the above range according to the third temperature used, for example, when the third temperature is higher, the heat treatment time can be appropriately shortened. Depending on the type of carbon source, the heat treatment time also varies slightly, and those skilled in the art can adjust it according to the actual situation.

[0114] In some embodiments, the heating rate in S20 may be 1°C / min-10°C / min. However, the present application is not limited thereto, and the heating rate may be adjusted according to actual conditions. For example, the heating rate is 1°C / min-9°C / min, 1°C / min-8°C / min, 1°C / min-7°C / min, 1°C / min-6°C / min, 1°C / min-5°C / min, 1°C / min-4°C / min, 1°C / min-3°C / min, 1°C / min-2°C / min, 2°C / min-10°C / min, 3°C / min-9°C / min, 4°C / min-8°C / min, or 5°C / min-7°C / min.

[0115] In some embodiments, the heating rate in S30 may be 1°C / min-5°C / min. However, the present application is not limited thereto, and the heating rate may be adjusted according to actual conditions. For example, the heating rate is 1°C / min-4°C / min, 1°C / min-3°C / min, 1°C / min-2°C / min, 2°C / min-5°C / min, 2°C / min-4°C / min, 2°C / min-3°C / min, 3°C / min-5°C / min, 3°C / min-4°C / min, or 4°C / min-5°C / min.

[0116] In some embodiments, the heating rate in S40 is 1°C / min-10°C / min. However, the present application is not limited thereto, and the heating rate can be adjusted according to actual conditions. For example, the heating rate is 1°C / min-9°C / min, 1°C / min-8°C / min, 1°C / min-7°C / min, 1°C / min-6°C / min, 1°C / min-5°C / min, 1°C / min-4°C / min, 1°C / min-3°C / min, 1°C / min-2°C / min, 2°C / min-10°C / min, 3°C / min-9°C / min, 4°C / min-8°C / min, or 5°C / min-7°C / min.

[0117] In some embodiments, after S20 and before S30, the preparation method further includes: S21, crushing the first intermediate product obtained in S20, for example, to the mm level. At this time, the agglomerated first intermediate product can be crushed to ensure that the first intermediate product can fully contact with air during subsequent processing. Of course, in some embodiments, this step can be omitted.

[0118] In some embodiments, after S30 and before S40, the preparation method further includes: S31, crushing the second intermediate product obtained in S30. For example, in some embodiments, the volume particle size Dv50 of the second intermediate product after crushing is 2μm-15μm, optionally 4μm-8μm. In some embodiments, the volume particle size Dv90 of the second intermediate product after crushing is 5μm-25μm, optionally 8μm-15μm. In some embodiments, the volume particle size Dv50 of the second intermediate product after crushing is 4μm-8μm and the volume particle size Dv90 is 8μm-15μm. Of course, in some embodiments, this step can be omitted.

[0119] In some embodiments, the preparation method further includes: S50, crushing the hard carbon obtained in S40. At this time, the agglomerated hard carbon can be crushed to meet the required particle size to facilitate the preparation of negative electrode slurry and negative electrode sheet. Of course, in some embodiments, this step can be omitted.

[0120] The present application has no particular limitation on the type of the carbon source. In some embodiments, the carbon source includes one or more of a polymer, a resin, and a biomass material.

[0121] As an example, the polymer includes one or more of polyaniline and polypyrrole.

[0122] As an example, the resin includes one or more of phenolic resin and epoxy resin. Optionally, the phenolic resin includes one or more of phenol formaldehyde resin, m-diphenol formaldehyde resin, p-diphenol formaldehyde resin, and phenol furfural resin.

[0123] As an example, the biomass material includes one or more of starch, glucose, fructose, maltose, sucrose, cellulose, hemicellulose, and lignin. Alternatively, the starch includes one or more of cereal starch, potato starch, and legume starch. Examples of the cereal starch may include, but are not limited to, one or more of corn starch, rice starch, millet starch, sorghum starch, wheat starch, oat starch, buckwheat starch, and rye starch; examples of the legume starch may include, but are not limited to, one or more of tapioca starch, potato starch, sweet potato starch, yam starch, and taro starch; and examples of the legume starch may include, but are not limited to, one or more of mung bean starch, broad bean starch, pea starch, and cowpea starch.

[0124] Optionally, the carbon source is selected from biomass materials. Furthermore, the carbon source is selected from one or more of starch, glucose, fructose, maltose, sucrose, cellulose, hemicellulose, and lignin. The above-mentioned biomass materials have a small number of heteroatoms (atoms other than O and H), a high carbon content, and a low price, which can avoid the influence of heteroatoms on the hard carbon preparation process. At the same time, the above-mentioned biomass materials can be cross-linked at a lower temperature, which is conducive to forming a more stable skeleton structure, and then obtaining hard carbon with a suitable pore structure.

[0125] In the present application, the biomass material can be directly purchased from commercial sources, or can be obtained by extraction from plants.

[0126] In some embodiments, the preparation method of the hard carbon includes the following steps: providing a carbon source, which is a biomass material, optionally starch; heat treating the carbon source in an inert atmosphere at less than or equal to 300°C for 4h-60h to obtain a first intermediate product; heat treating the obtained first intermediate product in an air atmosphere at less than 400°C for 1h-12h to obtain a second intermediate product; carbonizing the obtained second intermediate product in an inert atmosphere at 1000°C-1600°C for 1h-12h to obtain hard carbon.

[0127] In some embodiments, the preparation method of the hard carbon includes the following steps: providing a carbon source, which is a biomass material, optionally starch; heat treating the carbon source in an inert atmosphere at less than or equal to 300°C for 4h-60h to obtain a first intermediate product; heat treating the obtained first intermediate product in an air atmosphere at less than 400°C for 1h-12h to obtain a second intermediate product; crushing the obtained second intermediate product to a volume particle size Dv50 of 2μm-15μm and / or Dv90 of 5μm-25μm, and then carbonizing it in an inert atmosphere at 1000°C-1600°C for 1h-12h to obtain hard carbon.

[0128] secondary batteries

[0129] The third aspect of the embodiment of the present application provides a secondary battery. The present application has no particular restrictions on the type of secondary battery. For example, the secondary battery can be a lithium-ion battery, a sodium-ion battery, etc. In particular, the secondary battery is a sodium-ion secondary battery. Typically, a secondary battery includes a positive electrode sheet, a negative electrode sheet, and an electrolyte. During the charge and discharge process of the secondary battery, active ions are embedded and extracted back and forth between the positive electrode sheet and the negative electrode sheet, and the electrolyte plays a role in conducting active ions between the positive electrode sheet and the negative electrode sheet.

[0130] [Negative electrode]

[0131] In some embodiments, the negative electrode plate includes a negative electrode current collector and a negative electrode film layer disposed on at least one surface of the negative electrode current collector. For example, the negative electrode current collector has two surfaces that are opposite to each other in its thickness direction, and the negative electrode film layer is disposed on either or both of the two opposing surfaces of the negative electrode current collector.

[0132] In some embodiments, the negative electrode film layer includes at least one of the hard carbon described in any embodiment of the first aspect of the embodiment of the present application and the hard carbon prepared by the method described in any embodiment of the second aspect of the embodiment of the present application.

[0133] In some embodiments, the negative electrode film layer may further include other negative electrode active materials in addition to the above-mentioned hard carbon. In some embodiments, the other negative electrode active materials include but are not limited to at least one of natural graphite, artificial graphite, soft carbon, silicon-based materials, tin-based materials, and lithium titanate. The silicon-based material may include at least one of elemental silicon, silicon oxide, silicon-carbon composite, silicon-nitrogen composite, and silicon alloy material. The tin-based material may include at least one of elemental tin, tin oxide, and tin alloy material. The present application is not limited to these materials, and other conventionally known materials that can be used as negative electrode active materials for secondary batteries may also be used.

[0134] In some embodiments, the negative electrode film layer may further optionally include a negative electrode conductive agent. This application does not specifically limit the type of the negative electrode conductive agent. By way of example, the negative electrode conductive agent may include at least one of superconducting carbon, conductive graphite, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers. In some embodiments, the mass percentage of the negative electrode conductive agent, based on the total mass of the negative electrode film layer, is ≤5%.

[0135] In some embodiments, the negative electrode film layer may further optionally include a negative electrode binder. The present application does not particularly limit the type of the negative electrode binder. As an example, the negative electrode binder may include at least one of styrene-butadiene rubber (SBR), water-soluble unsaturated resin SR-1B, water-based acrylic resin (e.g., polyacrylic acid PAA, polymethacrylic acid PMAA, sodium polyacrylate PAAS), polyacrylamide (PAM), polyvinyl alcohol (PVA), sodium alginate (SA), and carboxymethyl chitosan (CMCS). In some embodiments, the mass percentage of the negative electrode binder is ≤5% based on the total mass of the negative electrode film layer.

[0136] In some embodiments, the negative electrode film layer may optionally include other additives. For example, the other additives may include a thickener, such as sodium carboxymethyl cellulose (CMC-Na), a PTC thermistor material, or the like. In some embodiments, the other additives may comprise ≤2% by weight based on the total weight of the negative electrode film layer.

[0137] In some embodiments, the negative electrode current collector may be a metal foil or a composite current collector. As an example of a metal foil, copper foil may be used. The composite current collector may include a polymer material base layer and a metal material layer formed on at least one surface of the polymer material base layer. As an example, the metal material may be selected from at least one of copper, copper alloy, nickel, nickel alloy, titanium, titanium alloy, silver, and silver alloy. As an example, the polymer material base layer may be selected from polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.

[0138] The negative electrode film layer is typically formed by coating a negative electrode slurry onto a negative electrode current collector, drying, and cold pressing. The negative electrode slurry is typically formed by dispersing the negative electrode active material, an optional conductive agent, an optional binder, and other optional additives in a solvent and stirring until uniformly mixed. The solvent may be, but is not limited to, N-methylpyrrolidone (NMP) or deionized water.

[0139] The negative electrode plate does not exclude other additional functional layers in addition to the negative electrode film layer. For example, in certain embodiments, the negative electrode plate described herein further includes a conductive primer layer (e.g., composed of a conductive agent and a binder) sandwiched between the negative electrode current collector and the negative electrode film layer and disposed on the surface of the negative electrode current collector. In other embodiments, the negative electrode plate described herein further includes a protective layer covering the surface of the negative electrode film layer.

[0140] [Positive electrode]

[0141] In some embodiments, the positive electrode sheet includes a positive electrode current collector and a positive electrode film layer disposed on at least one surface of the positive electrode current collector. For example, the positive electrode current collector has two surfaces that are opposite to each other in its thickness direction, and the positive electrode film layer is disposed on either or both of the two opposing surfaces of the positive electrode current collector.

[0142] The positive electrode current collector may be a metal foil or a composite current collector. As an example of a metal foil, aluminum foil may be used. The composite current collector may include a polymer material base layer and a metal material layer formed on at least one surface of the polymer material base layer. As an example, the metal material may be selected from one or more of aluminum, aluminum alloy, nickel, nickel alloy, titanium, titanium alloy, silver, and silver alloy. As an example, the polymer material base layer may be selected from polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.

[0143] The positive electrode film layer generally comprises a positive electrode active material, an optional binder and an optional conductive agent. The positive electrode film layer is generally formed by coating a positive electrode slurry on the positive electrode current collector, drying and cold pressing. The positive electrode slurry is generally formed by dispersing a positive electrode active material, an optional conductive agent, an optional binder and any other components in a solvent and stirring evenly. The solvent may be N-methylpyrrolidone (NMP), but is not limited thereto. As an example, the binder for the positive electrode film layer may include one or more of polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), vinylidene fluoride-tetrafluoroethylene-propylene terpolymer, vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene terpolymer, tetrafluoroethylene-hexafluoropropylene copolymer, and fluorine-containing acrylate resin. As an example, the conductive agent for the positive electrode film layer includes one or more of superconducting carbon, conductive graphite, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0144] The positive electrode active material may be a positive electrode active material for a secondary battery known in the art.

[0145] When the secondary battery of the present application is a lithium-ion battery, the positive electrode active material may include one or more of a lithium transition metal oxide, an olivine-structured lithium-containing phosphate, and their respective modified compounds. Examples of lithium transition metal oxides may include, but are not limited to, one or more of lithium cobalt oxide, lithium nickel oxide, lithium manganese oxide, lithium nickel cobalt oxide, lithium manganese cobalt oxide, lithium nickel manganese oxide, lithium nickel cobalt manganese oxide, lithium nickel cobalt aluminum oxide, and their modified compounds. Examples of lithium phosphates with an olivine structure may include, but are not limited to, one or more of lithium iron phosphate, a composite material of lithium iron phosphate and carbon, lithium manganese phosphate, a composite material of lithium manganese phosphate and carbon, lithium manganese iron phosphate, a composite material of lithium manganese iron phosphate and carbon, and their respective modified compounds. The present application is not limited to these materials, and other conventionally known materials that can be used as positive electrode active materials for lithium ion batteries may also be used. These positive electrode active materials may be used alone or in combination of two or more.

[0146] In some embodiments, in order to further improve the energy density of the secondary battery, the positive electrode active material for the lithium ion battery may include one or more of the lithium transition metal oxides and modified compounds thereof shown in Formula 1,

[0147] Li a Ni b Co c M d O e A f Formula 1,

[0148] In formula 1, 0.8≤a≤1.2, 0.5≤b<1, 0<c<1, 0<d<1, 1≤e≤2, 0≤f≤1, M is selected from one or more of Mn, Al, Zr, Zn, Cu, Cr, Mg, Fe, V, Ti and B, and A is selected from one or more of N, F, S and Cl.

[0149] As an example, the positive active material for lithium ion batteries may include LiCoO2, LiNiO2, LiMnO2, LiMn2O4, LiNi 1 / 3 Co 1 / 3 Mn 1 / 3 O2(NCM333),LiNi 0.5 Co 0.2 Mn 0.3 O2(NCM523), LiNi 0.6 Co 0.2 Mn 0.2 O2(NCM622),LiNi 0.8 Co 0.1 Mn 0.1 O2(NCM811), LiNi 0.85 Co 0.15 Al 0.05 One or more of O2, LiFePO4, and LiMnPO4.

[0150] When the secondary battery of the present application is a sodium ion battery, in particular, when it is a sodium ion secondary battery, the positive electrode active material may include one or more of a sodium-containing transition metal oxide, a polyanion material (such as phosphate, fluorophosphate, pyrophosphate, sulfate, etc.), and a Prussian blue material. The present application is not limited to these materials, and other conventionally known materials that can be used as positive electrode active materials for sodium ion batteries may also be used. These positive electrode active materials may be used alone or in combination of two or more.

[0151] As an example, the positive active material for sodium ion batteries may include NaFeO2, NaCoO2, NaCrO2, NaMnO2, NaNiO2, NaNi 1 / 2 Ti 1 / 2 O2、NaNi 1 / 2 Mn 1 / 2 O2、Na 2 / 3 Fe 1 / 3 Mn 2 / 3 O2、NaNi 1 / 3 Co 1 / 3 Mn 1 / 3 O2, NaFePO4, NaMnPO4, NaCoPO4, Prussian blue materials, general formula A a M b(PO4) c O x Y 3-x Materials (wherein A is selected from H + 、Li + 、Na + , K + and NH4 + M is a transition metal cation, optionally one or more of V, Ti, Mn, Fe, Co, Ni, Cu and Zn, Y is a halogen anion, optionally one or more of F, Cl and Br, 0<a≤4, 0<b≤2, 1≤c≤3, 0≤x≤2).

[0152] In the present application, the modified compounds of the above-mentioned positive electrode active materials may be the positive electrode active materials subjected to doping modification or surface coating modification.

[0153] The positive electrode sheet of the present application does not exclude other additional functional layers in addition to the positive electrode film layer. For example, in some embodiments, the positive electrode sheet of the present application further includes a conductive primer layer (e.g., composed of a conductive agent and a binder) sandwiched between the positive electrode current collector and the positive electrode film layer and disposed on the surface of the positive electrode current collector. In other embodiments, the positive electrode sheet of the present application further includes a protective layer covering the surface of the positive electrode film layer.

[0154] [Electrolytes]

[0155] The present application has no particular limitation on the type of the electrolyte, and the electrolyte can be selected according to actual needs. For example, the electrolyte can be selected from at least one of a solid electrolyte and a liquid electrolyte (ie, an electrolyte solution).

[0156] In some embodiments, the electrolyte is an electrolyte solution comprising an electrolyte salt and a solvent.

[0157] The type of the electrolyte salt is not particularly limited and can be selected according to actual needs.

[0158] When the secondary battery of the present application is a lithium ion battery, as an example, the electrolyte salt may include at least one of lithium hexafluorophosphate (LiPF6), lithium tetrafluoroborate (LiBF4), lithium perchlorate (LiClO4), lithium hexafluoroarsenate (LiAsF6), lithium bis(fluorosulfonyl)imide (LiFSI), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), lithium trifluoromethanesulfonate (LiTFS), lithium difluorooxalatoborate (LiDFOB), lithium dioxalatoborate (LiBOB), lithium difluorophosphate (LiPO2F2), lithium difluorobis(oxalatophosphate) (LiDFOP), and lithium tetrafluorooxalatophosphate (LiTFOP).

[0159] When the secondary battery of the present application is a sodium ion battery, in particular, a sodium ion secondary battery, the electrolyte salt may include one or more of sodium hexafluorophosphate (NaPF6), sodium tetrafluoroborate (NaBF4), sodium perchlorate (NaClO4), sodium hexafluoroarsenate (NaAsF6), sodium bis(fluorosulfonyl)imide (NaFSI), sodium bis(trifluoromethanesulfonyl)imide (NaTFSI), sodium trifluoromethanesulfonate (NaTFS), sodium difluorooxalatoborate (NaDFOB), sodium dioxalatoborate (NaBOB), sodium difluorophosphate (NaPO2F2), sodium difluorobis(oxalatophosphate) (NaDFOP), and sodium tetrafluorooxalatophosphate (NaTFOP).

[0160] The type of the solvent is not specifically limited and can be selected according to actual needs. In some embodiments, as an example, the solvent may include ethylene carbonate (EC), propylene carbonate (PC), ethyl methyl carbonate (EMC), diethyl carbonate (DEC), dimethyl carbonate (DMC), dipropyl carbonate (DPC), methyl propyl carbonate (MPC), ethyl propyl carbonate (EPC), butylene carbonate (BC), fluoroethylene carbonate (FEC), methyl formate (MF), methyl acetate (MA), ethyl acetate (EA), propyl acetate (PA), methyl propionate (MP), ethyl propionate (EP), propyl propionate (PP), methyl butyrate (MB), ethyl butyrate (EB), 1,4-butyrolactone (GBL), sulfolane (SF), dimethyl sulfone (MSM), ethyl methyl sulfone (EMS) and diethyl sulfone (ESE). One or more.

[0161] In some embodiments, the electrolyte may further include additives. For example, the additives may include negative electrode film-forming additives, positive electrode film-forming additives, or additives capable of improving certain battery properties, such as additives that improve battery overcharge performance, additives that improve battery high-temperature performance, and additives that improve battery low-temperature power performance.

[0162] [Isolation film]

[0163] Secondary batteries using electrolytes, as well as some using solid-state electrolytes, also include a separator. This separator is positioned between the positive and negative electrode sheets to provide isolation. This application does not specifically limit the type of separator; any known porous separator with good chemical and mechanical stability can be used.

[0164] In some embodiments, the material of the isolation membrane can be selected from one or more of glass fiber, non-woven fabric, polyethylene, polypropylene, and polyvinylidene fluoride. The isolation membrane can be a single-layer film or a multi-layer composite film. When the isolation membrane is a multi-layer composite film, the materials of each layer can be the same or different.

[0165] In some embodiments, the positive electrode sheet, the separator, and the negative electrode sheet can be formed into an electrode assembly through a winding process or a lamination process.

[0166] In some embodiments, the secondary battery may include an outer packaging that can be used to encapsulate the electrode assembly and the electrolyte.

[0167] In some embodiments, the outer packaging of the secondary battery can be a hard shell, such as a hard plastic shell, an aluminum shell, a steel shell, etc. The outer packaging of the secondary battery can also be a soft package, such as a bag-type soft package. The material of the soft package can be plastic, such as one or more of polypropylene (PP), polybutylene terephthalate (PBT), polybutylene succinate (PBS), etc.

[0168] The present application has no particular restrictions on the shape of the secondary battery, which can be cylindrical, square or any other shape. Figure 2 The secondary battery 5 is a square structure as an example.

[0169] In some embodiments, as Figure 3 As shown, the outer packaging may include a shell 51 and a cover plate 53. The shell 51 may include a bottom plate and side plates connected to the bottom plate, and the bottom plate and the side plates enclose a receiving cavity. The shell 51 has an opening connected to the receiving cavity, and the cover plate 53 is used to cover the opening to close the receiving cavity. The positive electrode sheet, the negative electrode sheet and the isolation membrane can be formed into an electrode assembly 52 through a winding process or a lamination process. The electrode assembly 52 is encapsulated in the receiving cavity. The electrolyte is impregnated in the electrode assembly 52. ​​The number of electrode assemblies 52 contained in the secondary battery 5 can be one or more, which can be adjusted according to demand.

[0170] The preparation method of the secondary battery of the present application is well known. In some embodiments, a positive electrode sheet, a separator, a negative electrode sheet, and an electrolyte can be assembled to form a secondary battery. As an example, the positive electrode sheet, separator, and negative electrode sheet can be formed into an electrode assembly through a winding process or a lamination process. The electrode assembly is placed in an outer packaging, dried, and then injected with electrolyte. The secondary battery is then vacuum packaged, allowed to stand, formed, and shaped.

[0171] In some embodiments of the present application, the secondary batteries according to the present application can be assembled into a battery module. The battery module can contain multiple secondary batteries, and the specific number can be adjusted according to the application and capacity of the battery module.

[0172] Figure 4 FIG. 4 is a schematic diagram of a battery module 4 as an example. Figure 4As shown, in the battery module 4, the plurality of secondary batteries 5 can be arranged in sequence along the length direction of the battery module 4. Of course, they can also be arranged in any other manner. Further, the plurality of secondary batteries 5 can be fixed by fasteners.

[0173] Optionally, the battery module 4 may further include a housing having a receiving space, and the plurality of secondary batteries 5 are received in the receiving space.

[0174] In some embodiments, the battery modules described above may also be assembled into a battery pack, and the number of battery modules contained in the battery pack may be adjusted according to the application and capacity of the battery pack.

[0175] Figure 5 and Figure 6 FIG. 1 is a schematic diagram of a battery pack 1 as an example. Figure 5 and Figure 6 As shown, a battery pack 1 may include a battery box and multiple battery modules 4 disposed within the battery box. The battery box includes an upper box body 2 and a lower box body 3. The upper box body 2 is used to cover the lower box body 3 and form an enclosed space for accommodating the battery modules 4. The multiple battery modules 4 can be arranged in any manner within the battery box.

[0176] Electrical devices

[0177] The embodiments of the present application also provide an electric device, which includes at least one of the secondary battery, battery module, or battery pack of the present application. The secondary battery, battery module, or battery pack can be used as a power source for the electric device, and can also be used as an energy storage unit for the electric device. The electric device can be, but is not limited to, a mobile device (such as a mobile phone, a laptop computer, etc.), an electric vehicle (such as a pure electric vehicle, a hybrid electric vehicle, a plug-in hybrid electric vehicle, an electric bicycle, an electric scooter, an electric golf cart, an electric truck, etc.), an electric train, a ship, a satellite, an energy storage system, etc.

[0178] The electrical device may select a secondary battery, a battery module or a battery pack according to its usage requirements.

[0179] Figure 7 The diagram is a schematic diagram of an exemplary electric device. This device is a pure electric vehicle, a hybrid electric vehicle, or a plug-in hybrid electric vehicle. To meet the high power and high energy density requirements of this device, a battery pack or battery module can be used.

[0180] As another example, the electric device may be a mobile phone, a tablet computer, a laptop computer, etc. Such an electric device is usually required to be lightweight and thin, and may use a secondary battery as a power source.

[0181] Example

[0182] The following examples describe the present disclosure in more detail. These examples are intended to be illustrative only, as various modifications and variations within the scope of the present disclosure will be apparent to those skilled in the art. Unless otherwise stated, all parts, percentages, and ratios reported in the following examples are by mass, and all reagents used in the examples are commercially available or synthesized according to conventional methods and can be used directly without further processing. The instruments used in the examples are commercially available.

[0183] Example 1

[0184] Corn starch as a carbon source is placed in a tubular furnace, and the temperature is increased to 300°C (first temperature T1) at a rate of 5°C / min under a nitrogen atmosphere, and then the mixture is kept warm for 18 hours (heat treatment time t1) to obtain a first intermediate product; the nitrogen atmosphere is changed to an air atmosphere, and the temperature is then increased to 300°C (second temperature T2) at a rate of 3°C / min, and the mixture is kept warm for 2 hours (heat treatment time t2) to obtain a second intermediate product; the obtained second intermediate product is crushed to a volume particle size Dv50 of 4μm-8μm and Dv90 of 8μm-15μm, and then the temperature is increased to 1100°C (third temperature T3) at a rate of 5°C / min under a nitrogen atmosphere, and the mixture is kept warm for 12 hours (heat treatment time t3) to obtain hard carbon.

[0185] Example 2-20

[0186] The preparation method of hard carbon is similar to that of Example 1, except that the preparation process parameters of hard carbon are adjusted. See Table 1 for details.

[0187] Comparative Example 1

[0188] Corn starch as a carbon source is placed in a tubular furnace, heated to 240°C at a rate of 3°C / min under a nitrogen atmosphere, and then heat-treated for 36 hours to obtain a first intermediate product; the obtained first intermediate product is crushed to a volume particle size Dv50 of 4μm-8μm and Dv90 of 8μm-15μm, and then heated to 1200°C at a rate of 5°C / min under a nitrogen atmosphere, and then heat-treated for 12 hours to obtain hard carbon.

[0189] Comparative Example 2

[0190] Corn starch as a carbon source is placed in a tubular furnace, heated to 240°C at a rate of 3°C / min in an air atmosphere, and then heat-treated for 36 hours to obtain a first intermediate product; the obtained first intermediate product is crushed to a volume particle size Dv50 of 4μm-8μm and Dv90 of 8μm-15μm, and then heated to 1200°C at a rate of 5°C / min in a nitrogen atmosphere, and then heat-treated for 12 hours to obtain hard carbon.

[0191] Comparative Example 3

[0192] Corn starch as a carbon source was placed in a tube furnace, and after being heated to 300°C at a rate of 3°C / min under an air atmosphere, heat treatment was carried out for 2h, to obtain a first intermediate product; the obtained first intermediate product was crushed to a volume particle size Dv50 of 4-8μm and Dv90 of 8-15μm, and then heat treatment was carried out for 12h after being heated to 1200°C at a rate of 5°C / min under a nitrogen atmosphere, to obtain hard carbon.

[0193] Comparative Examples 4-6

[0194] The preparation method of the hard carbon was similar to that of Example 1, except that the preparation process parameters of the hard carbon were adjusted, and the specific details are shown in Table 1.

[0195] The hard carbon prepared in each example and comparative example was mixed with the binder styrene-butadiene rubber (SBR), thickening agent sodium carboxymethyl cellulose (CMC-Na), and conductive agent carbon black in a mass ratio of 96.2:1.8:1.2:0.8 in an appropriate amount of solvent deionized water to form a uniform negative electrode slurry; the negative electrode slurry was uniformly coated on the surface of the negative electrode current collector copper foil, and after drying in an oven, it was ready for use. Ethylene carbonate (EC), methyl ethyl carbonate (EMC), and diethyl carbonate (DEC) were mixed in a volume ratio of 1:1:1 to obtain an organic solvent, and then NaPF6 was dissolved in the above organic solvent to prepare an electrolyte with a concentration of 1mol / L. Then, with a piece of metallic sodium as the counter electrode and a polyethylene (PE) film as the separator, a CR2430 type button cell was assembled in an argon-protected glove box.

[0196] At 25°C, the button cell prepared in each example and comparative example was first discharged at a current density of 10mA / g to 0V, and the first circle discharge capacity of the button cell was recorded; then it was charged at a current density of 10mA / g to 2.0V, and the first circle charge capacity of the button cell was recorded.

[0197] The specific capacity of the hard carbon (mAh / g) = the first circle charge capacity of the button cell / the mass of the hard carbon.

[0198] The first cycle coulombic efficiency of the hard carbon (%) = the first circle charge capacity of the button cell / the first circle discharge capacity of the button cell x 100%.

[0199] The test results of Examples 1-20 and Comparative Examples 1-6 are shown in Table 2.

[0200] Figure 8 is a scanning electron microscope (SEM) image of the hard carbon prepared in Example 2, from which Figure 8 It can be seen that the hard carbon obtained by the preparation method of the present application has a regular morphology and uniform size.

[0201] Figure 9 and Figure 10 They are the nitrogen adsorption isotherms of the hard carbon prepared in Example 2 and Comparative Example 1 measured at a temperature of 77K.

[0202] like Figure 9 As shown, the nitrogen adsorption isotherm of the hard carbon prepared in Example 2 measured at a temperature of 77K satisfies Figure 1 The type I adsorption isotherm in the nitrogen relative pressure P / P0 is 10 -8 The total amount of nitrogen adsorption between 0.05 and 0.035 is 117.1 cm 3 (STP) / g, the total amount of nitrogen adsorption when the nitrogen relative pressure P / P0 is between 0.035 and 1 is 15.5cm 3 (STP) / g, and satisfies V2 / V1≤0.20. From the test results in Table 2, it can be seen that the gram capacity of the hard carbon prepared in Example 2 is 403 mAh / g and the first coulombic efficiency is 85.4%. Although the mechanism is not clear, the inventors speculate that one possible reason is that the pore structure of the hard carbon prepared in Example 2 is suitable and well-developed, and the hard carbon can provide more active ion storage sites, thereby having a higher gram capacity; at the same time, due to the well-developed pore structure of the hard carbon, the active ions stored in the active ion storage sites are easy to escape, so the first coulombic efficiency of the hard carbon is also high.

[0203] Comparative Example 1 is to subject corn starch to a low-temperature heat treatment process in an inert atmosphere and then directly perform a high-temperature carbonization treatment in an inert atmosphere. Figure 10 As shown, the nitrogen adsorption isotherm of the hard carbon prepared in Comparative Example 1 measured at a temperature of 77K does not meet Figure 1 The adsorption isotherm is similar to the type I adsorption isotherm Figure 1 The type II adsorption isotherm of the prepared hard carbon measured at a temperature of 77K does not meet the requirements of V2 / V1≤0.20 and 20≤V1≤150. From the test results in Table 2, it can be seen that the gram capacity of the hard carbon prepared in Comparative Example 1 is only 251mAh / g, and the first coulombic efficiency is only 77.7%. The possible reason is that the hard carbon prepared in Comparative Example 1 has fewer microporous structures and cannot provide more active ion storage sites, and the utilization rate of the only active ion storage sites is also low. At the same time, the proportion of larger pores (such as mesopores or macroporous structures) in the hard carbon prepared in Comparative Example 1 is too high. When the hard carbon is prepared into a secondary battery, the proportion of the electrolyte infiltration area inside the hard carbon is too high, which further reduces the utilization rate of the active ion storage sites.

[0204] In Comparative Examples 2 and 3, corn starch is subjected to a low-temperature heat treatment process in an air atmosphere and then directly subjected to a high-temperature carbonization process in an inert atmosphere. The hard carbons prepared in these examples cannot simultaneously have a high gram capacity and a high first coulombic efficiency. This may be because the hard carbons prepared in Comparative Examples 2 and 3 also have fewer microporous structures, which cannot provide many active ion storage sites, and the utilization rate of the few active ion storage sites is also low. At the same time, the proportion of larger pores in the hard carbons prepared in Comparative Examples 2 and 3 is too high. When the hard carbons are prepared into secondary batteries, the proportion of the electrolyte-wetted area inside the hard carbons is too high, further reducing the utilization rate of the active ion storage sites.

[0205] Comparative Examples 4-6 simultaneously subject corn starch to a low-temperature heat treatment process in an inert atmosphere, a secondary low-temperature heat treatment process in an air atmosphere, and a high-temperature carbonization treatment process in an inert atmosphere. However, the heat treatment temperature and the heat treatment time are unreasonable, resulting in the adsorption isotherms of the prepared hard carbon failing to simultaneously satisfy V2 / V1≤0.20 and 20≤V1≤150, and thus the prepared hard carbon failing to simultaneously have a high gram capacity and a high first coulombic efficiency.

[0206] In Comparative Example 4, the hard carbon was heat-treated in air for too long during preparation. This resulted in excessive pore formation in the hard carbon, which in turn led to collapse of the pore structure and pore merging. This increased the number and proportion of larger pores within the hard carbon. When the hard carbon was used to make a secondary battery, the electrolyte-wetted area within the hard carbon increased, reducing the utilization of active ion storage sites. This made it difficult for the hard carbon to achieve both high specific capacity and high initial coulombic efficiency.

[0207] Comparative Example 5 employed an excessively low third temperature, T3, during the preparation of the hard carbon. This resulted in a large number of extremely small pores in the prepared hard carbon. These pores were weak and easily damaged or collapsed, leading to pore merging. This increased the number and proportion of larger pores within the hard carbon. When the hard carbon was fabricated into a secondary battery, the proportion of electrolyte-wetted areas within the hard carbon increased, reducing the utilization of active ion storage sites. Consequently, the hard carbon struggled to achieve high specific capacity and initial coulombic efficiency.

[0208] In Comparative Example 6, an excessively high third temperature T3 is used when preparing hard carbon. The graphite microcrystalline structure formed during the carbonization treatment is prone to micro-movement, which forces the pore structure of the hard carbon to be adjusted and significantly reduces the number of pore structures. As a result, it is difficult for the hard carbon to have both a high gram capacity and a high first coulombic efficiency.

[0209] From the test results of Examples 1-20, it can be seen that when the nitrogen adsorption isotherm of hard carbon measured at 77K satisfies both V2 / V1≤0.20 and 20≤V1≤150, the hard carbon has both high capacity and first coulombic efficiency.

[0210] The test results of Examples 1-20 also show that when the nitrogen adsorption isotherm of hard carbon measured at 77K further satisfies 0.08≤V2 / V1≤0.20, 70≤V1≤150 and 6≤V2≤30, the capacity and first coulombic efficiency of the hard carbon are further improved.

[0211] It should be noted that the present application is not limited to the above-mentioned embodiments. The above-mentioned embodiments are merely examples, and any embodiments having substantially the same structure and effect as the technical concept within the scope of the present application are all included in the technical scope of the present application. In addition, without departing from the scope of the present application, any other embodiments that can be conceived by those skilled in the art and that combine some of the constituent elements in the embodiments are also included in the scope of the present application.

[0212]

[0213] Table 2

[0214]

Claims

1. A hard carbon, wherein the nitrogen relative pressure P / P0 of the hard carbon in the nitrogen adsorption isotherm measured at a temperature of 77K is between 10 -8 The total amount of nitrogen adsorption between 0.01 and 0.035 is V1 cm 3 (STP) / g, the total amount of nitrogen adsorbed when the relative pressure of nitrogen P / P0 is between 0.035 and 1 is V2 cm 3 (STP) / g, the hard carbon satisfies: V2 / V1≤0.20, and 20≤V1≤150, wherein, P represents the true pressure of nitrogen, and P0 represents the saturated vapor pressure of nitrogen at a temperature of 77K.

2. The hard carbon according to claim 1, wherein 50≤V1≤150。 3. The hard carbon according to claim 2, wherein 70≤V1≤150。 4. The hard carbon according to claim 1, wherein 0.05≤V2 / V1≤0.

20.

5. The hard carbon according to claim 4, wherein 0.08≤V2 / V1≤0.

20.

6. The hard carbon according to any one of claims 1 to 5, wherein 4≤V2≤30。 7. The hard carbon according to claim 6, wherein 6≤V2≤30。 8. The hard carbon according to claim 1, wherein In the Raman spectrum of the hard carbon, I d / I g 1.20-1.32, I d Indicates that the Raman shift is 1350±50cm -1 d peak intensity in the range, I g Indicates that the Raman shift is 1580±50cm -1 g peak intensity in the range.

9. The hard carbon according to claim 1 or 8, wherein In the X-ray diffraction spectrum of the hard carbon, the 2θ value corresponding to the 002 peak is between 22° and 24°.

10. The hard carbon according to claim 1, wherein The hard carbon satisfies at least one of the following conditions (1) to (5): (1) The volume particle size Dv50 of the hard carbon is 2 μm-15 μm; (2) The volume particle size Dv90 of the hard carbon is 5 μm-25 μm; (3) The specific surface area of ​​the hard carbon is less than or equal to 5m 2 / g; (4) The powder compaction density of the hard carbon under a force of 50,000 N is 0.96 g / cm 3 -1.05g / cm 3 ; (5) The tap density of the hard carbon is 0.80 g / cm 3 -0.95g / cm 3 .

11. The hard carbon according to claim 10, wherein The hard carbon satisfies at least one of the following conditions (1) to (3): (1) The volume particle size Dv50 of the hard carbon is 4 μm-8 μm; (2) The volume particle size Dv90 of the hard carbon is 8 μm to 15 μm; (3) The specific surface area of ​​the hard carbon is 0.5 m 2 / g-5m 2 / g.

12. A method for preparing hard carbon, comprising the following steps: S10, providing a carbon source, wherein the carbon source includes one or more of a polymer, a resin, and a biomass material; S20, heat-treating the carbon source in an inert atmosphere at a first temperature T1 for a time t1 to obtain a first intermediate product; S30, heat-treating the obtained first intermediate product in an air atmosphere at a second temperature T2 for a time t2 to obtain a second intermediate product; S40, carbonizing the obtained second intermediate product in an inert atmosphere at a third temperature T3 for a time t3 to obtain hard carbon, wherein T1 is ≤ 300°C, T2 is < 400°C, T3 is 1000°C-1600°C, t1 is 4h-60h, t2 is 1h-12h, and t3 is 1h-12h; in, In the nitrogen adsorption isotherm of the hard carbon measured at a temperature of 77K, the nitrogen relative pressure P / P0 is 10 -8 The total amount of nitrogen adsorption between 0.01 and 0.035 is V1 cm 3 (STP) / g, the total amount of nitrogen adsorbed when the relative pressure of nitrogen P / P0 is between 0.035 and 1 is V2 cm 3 (STP) / g, the hard carbon satisfies: V2 / V1≤0.20, and 20≤V1≤150, wherein P represents the actual pressure of nitrogen, and P0 represents the saturated vapor pressure of nitrogen at a temperature of 77K.

13. The method according to claim 12, wherein: T1≤T2.

14. The method according to claim 12, wherein: T1 is 180℃-300℃.

15. The method according to claim 12, wherein: T2 is 200℃-395℃.

16. The method according to claim 15, wherein T2 is 270℃-380℃.

17. The method according to claim 12, wherein: 200℃≤T2≤270℃, and 10h≤t2≤12h.

18. The method according to claim 12, wherein: 270℃<T2<400℃, and 1h≤t2≤8h.

19. The method according to claim 12, wherein: T3 is 1000℃-1400℃.

20. The method according to claim 12, wherein t1 is 18h-60h.

21. The method according to claim 12, wherein The preparation method further comprises at least one of the following steps (a) to (c): (a) after S20 and before S30, further comprising: S21, crushing the first intermediate product obtained in S20; (b) after S30 and before S40, further comprising: S31, crushing the second intermediate product obtained in S30; (c) S50, crushing the hard carbon obtained in S40.

22. The method according to claim 21, wherein The volume particle size of the second intermediate product after crushing satisfies: Dv50 is 2μm-15μm, and / or Dv90 is 5μm-25μm.

23. The method according to claim 12, wherein The inert atmosphere is selected from one or more of nitrogen atmosphere and argon atmosphere.

24. The method according to claim 12, wherein The polymer includes one or more of polyaniline and polypyrrole.

25. The method according to claim 12, wherein The resin includes one or more of phenolic resin and epoxy resin.

26. The method according to claim 12, wherein The biomass material includes one or more of starch, glucose, fructose, maltose, sucrose, cellulose, hemicellulose, and lignin.

27. The method according to claim 26, wherein The starch includes one or more of cereal starch, potato starch, and bean starch.

28. A secondary battery comprising a negative electrode plate, wherein the negative electrode plate comprises the hard carbon according to any one of claims 1 to 11, or the hard carbon prepared by the method according to any one of claims 12 to 27.

29. An electric device comprising the secondary battery according to claim 28.

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